Feature/last insert id support pg

I usually use DBIx::Lite with Pg and most of the time it works well. But currently DBIx::Lite does not support Pg for returning row object after `update()`. I think `last_insert_id()` of DBI method will help `_autopk()` internal method of DBIx::Lite to support Pg. But I cannot sure that `last_insert_id()` is good or not rather than MySQL & SQLite's way. So i left them and just add for Pg.

If there is no problem to support Pg, I hope I could install next release from CPAN. ;-)